In recent years, image forming apparatus adopting the electrophotography system have been known that includes a primary transfer belt bearing and conveying a developer image having been formed on a plurality of image bearing members and a secondary transfer unit transferring the developer image borne by the primary transfer belt onto a paper sheet in a state of being in contact with pressure with the primary transfer belt. The secondary transfer unit is configured in such a manner as to be capable of being freely displaced between a pressing position and a separate position in relation to the primary transfer belt. Among such image forming apparatus is one using a pair of eccentric cams as a displacement mechanism for the secondary transfer unit that are in contact with either edge portion of the secondary transfer unit in width direction of the primary transfer belt.
Rotatory torque of the eccentric cam varies a great deal depending on its rotational angle. In conventional image forming apparatus, because a pair of eccentric cams of a shape identical to each other has been used, the relationship between the rotatory torque and the rotational angle has been the same within the pair of eccentric cams. This has caused the pair of eccentric cams to have the same rotational angle at which their rotatory torques become greatest, thereby a high-level load occurs in their drive system members including gears, an electromagnetic clutch, a shaft member supporting them and the like when the pair of eccentric cams rotates. Therefore, there have been problems that the electromagnetic clutch is prone to slip, that the shaft member is prone to damage, and that the gears are prone to tooth abrasion and damage.
Then, a displacement mechanism for the secondary transfer unit has been proposed that is configured in such a manner that only a first eccentric cam of the pair of cams is in contact with a rotating member which is a follower at a separation start position and at a maximum torque position, and that only a second eccentric cam is in contact with the rotating member at a contact-with-pressure start position (for example, refer to Patent Literature 1).